Field of the Invention
The invention concerns a method for controlling a table position of an examination table of a magnetic resonance system during a magnetic resonance measurement (data acquisition). Furthermore the invention concerns a device for controlling a table position of an examination table of a magnetic resonance system during a magnetic resonance measurement. The invention further relates to such a magnetic resonance system.
Description of the Prior Art
In a magnetic resonance system, the body to be examined is normally exposed to a relatively high basic magnetic field, for example of 3 or 7 tesla, with the use of a basic field magnet. In addition a magnetic field gradient is created with the use of a gradient system. Using a radio-frequency transmission system, radio-frequency excitation signals (RF signals) are then transmitted by suitable antennas, which results in the nuclear spins of particular atoms being resonantly excited by this radio-frequency field, so as to be tilted in a spatially resolved manner by a defined flip angle compared to the magnetic field lines of the basic magnetic field. This radio-frequency excitation or the resulting flip angle distribution is also referred to below as nuclear magnetization or “magnetization” for short. When the nuclear spins relax, radio-frequency signals known as magnetic resonance signals are emitted and are received by suitable receiving antennas, and then further processed. From the raw data acquired in this way, the desired image data can ultimately be reconstructed. The transmission of the radio-frequency signals for nuclear spin magnetization takes place, for example, by a so-called “whole-body coil” or “body coil”, or often using local coils positioned on the patient or test person.
The radio-frequency signals result in an overall radio-frequency load on the patient, which has to be restricted in accordance with legal requirements, because an excessive radio-frequency load could result in injury to the patient. Hence the radio-frequency load on the patient is generally calculated in advance while planning the radio-frequency pulses to be emitted and the radio-frequency pulses are selected such that a particular limit is not reached. In the following the RF load means a physiological load induced by the RF radiation and not the introduced RF energy as such. A typical measure for the radio-frequency load is the SAR value (SAR=Specific Absorption Rate), which indicates in watts/kg the biological load that is being applied to the patient by a particular radio-frequency pulse output. In other words, the SAR value describes the energy absorption behavior of the tissue, which results in warming of the tissue that is exposed to the radio-frequency pulses. For the overall SAR or RF load of a patient, a standardized limit of 4 watts/kg, for example, applies in the “first level” in accordance with the IEC standard. In addition, apart from the advance planning, the SAR load of the patient is continuously monitored during the examination using suitable safety devices in the magnetic resonance system and the data acquisition is changed or aborted if the SAR value goes above the permitted standards. Nevertheless, it makes sense for advance planning to be as accurate as possible in order to avoid aborting a measurement, since this would necessitate a new measurement.
For planning the RF pulse sequence, the user specifies a target magnetization, for example a desired spatially resolved flip angle distribution, to be employed as a target value within the target region. Crucial to the energy output during an MR examination is the voltage with which the radio-frequency pulses are transmitted, known as the RF transmitter voltage. It is determined in a separate MR scan, namely the AdjTra measurement (AdjTra being an abbreviation for adjust transmitter). To be more precise, during the AdjTra measurement the voltage is determined that is required in order to achieve an RF pulse that brings about a tilting of the resonantly excited atoms by a defined flip angle in particular tissue. This RF transmitter voltage varies considerably, both from patient to patient and very distinctly as a function of the position of the patient or of the table, on which the patient is positioned, within the RF resonator. Here a variation in the transmitter voltage by a factor of 1.5 is common when the position of the patient is changed by a few centimeters in the Z direction.
The SAR load can also be reduced by slowing the measurement. Thus the SAR is the limiting factor for a short measurement time in many clinical protocols. For this reason MR protocols are frequently created such that the permitted SAR limit values are just complied with. The result is that for patients with a high specific absorption or patients who because of their constitution can only tolerate a lower load, the SAR limit values are clearly exceeded in particular positions.
To prevent this, an SAR forecast is made by a so-called SAR solver after pre-setting the measurement, initially on the basis of adjustment parameters. If the load values determined in this way lie above the limit value, the measurement is aborted even before the start and the protocol parameters are changed. For example, the repetition time TR of the pulse sequences is extended or the number of slices recorded is reduced. In the latter case either a smaller FoV (field of view) is accepted or the resolution of the recorded image is reduced. As a further measure, a reduction can also conventionally be made in the flip angle, which is achieved by reducing the transmitter voltage. The flip angle can also be adjusted automatically. For example, before the measurement the user selects a minimum flip angle, up to which the flip angle can be automatically adjusted. If on the basis of the SAR forecast, it is now found necessary to reduce the flip angle, the system automatically changes the transmitter voltage, providing the corresponding flip angle is less than the predetermined minimum value. However, the result of these techniques is that parameters of the protocol are changed such that the result of the measurement no longer satisfies the original requirements in terms of image quality and patient coverage. Furthermore, because the measurement procedure is aborted at the start and the settings of the measurement protocol are changed, the examination sequence is interrupted and the time the patient spends in the MR scanner is increased.